ORIGINAL CONTRIBUTION Diffusing wave spectroscopy investigations of acid milk gels containing pectin A. Cucheval & R. R. Vincent & Y. Hemar & D. Otter & M. A. K. Williams Received: 15 September 2008 / Revised: 28 November 2008 / Accepted: 29 January 2009 / Published online: 24 February 2009 # Springer-Verlag 2009 Abstract The influence of the polysaccharide pectin on the gelation of acidified milk is studied in concentrated, undiluted, quiescent systems, primarily using diffusing wave spectroscopy. For pectins with a low degree of methylesterification (DM), interactions with milk-serum calcium yielded precipitated polysaccharide aggregates, even without acidification, that subsequently did not interact with casein micelles. However, high DM fine structures do not interact significantly with serum-calcium and absorb onto casein micelles as the pH is reduced below 5. A limited surface coverage of high DM pectin facilitates efficient bridging which enhances the rate of micelle aggregation and subsequent gelation and produces a clear signature in the shape of the measured MSD. The work highlights the fact that the behaviour of pectin in milk systems depends not only on the interaction of different polymeric fine structures with casein micelles, but also to a large extent on the interactions with calcium. Keywords Milk . Casein micelles . Pectin . Acidification . Diffusing wave spectroscopy Introduction Casein moieties in milk are assembled into micelles during biosynthesis and despite the fact that the detailed arrangement of the protein variants is complex, it is well established that these entities are stabilised in solution by kappa-casein molecules forming an entropy rich steric barrier at their surface [1]. Enzymatic cleavage or electrostatic collapse of this barrier destabilises the micelles and triggers assembly, yielding the formation of networks that consist primarily of aggregated casein micelles. While the micellar integrity depends in detail upon the nature of the environmental conditions employed in destabilisation, these systems never- theless tend to exhibit the microstructural appearance of particulate networks—a fact that has not escaped the attention of physicists interested in colloidal assembly per se [2, 3]. Thus, acid milk gels (those triggered by the collapse of the stabilising protein layer owing to changes in the polymeric charge brought about by lowering the pH) are built of a three dimensional network of chains and clusters of milk proteins that at a smaller scale retain some of the integrity of the particulate micellar form [4]. They do, however, form a heterogeneous and complex system. The heterogeneity manifests at two levels: in the network itself and in the presence of voids in the colloidal system. Acid milk gels have been extensively studied by bulk rheology [5] and more recently by microrheological techniques [2, 6–9]. In particular, diffusing wave spectroscopy (DWS), a non- invasive multiple scattering technique, has found consider- able utility in studying the underlying dynamics of these systems. Here, autocorrelation functions resulting from the intensity fluctuations of light that has been multiply scattered by the sample are analysed and information on the dynamics of the scatterers is thus extracted. In these systems the scattering is dominated by casein micelles and protein Colloid Polym Sci (2009) 287:695–704 DOI 10.1007/s00396-009-2012-5 A. Cucheval : R. R. Vincent : Y. Hemar : M. A. K. Williams (*) Institute of Fundamental Sciences, Massey University, Palmerston North, New Zealand e-mail: m.williams@massey.ac.nz R. R. Vincent : Y. Hemar : M. A. K. Williams MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand A. Cucheval : D. Otter Fonterra Research Centre, Palmerston North, New Zealand